Reconstruction of the Field Distribution by Measuring the Fundamental Passband Frequencies of the Rossendorf SRF-Gun Cavity

In contrast to the TESLA cavities the shapes and the mechanical stiffness of the four SRF-Gun cells differ from each other. Furthermore the axis field to achieve a “flat” surface field over all cells has a profile of 60% in the first and 100% in the TESLA like cells, respectively. Due to the different mechanical properties one tuner for the half cell and one for the three TESLA cells are assembled. Both of them are equipped to manipulate the field of the cavity during the adjustment of the tuner bias and later during operation. Hence it is very important to determine the fields inside the cleaned and closed cavity.
The poster presents a method that provides the field profile by measuring the passband frequencies based on the well known equivalent circuit for cell coupled cavities. At least one complete field and frequency measurement prior the last cleaning is necessary. The comparison with real measured values demonstrates the accuracy of the predicted field distribution within a range of 2%.